[SciPy-user] HDULists sharing HDU entries

Christopher Hanley chanley@stsci....
Fri Jul 11 20:36:20 CDT 2008


C. Allen wrote:
> I have been making a system for handling astronomical data, the actual 
> fits manipulation using pyfits. Please correct me if there is another 
> list this question would be more appropriate on, I did not see a pyfits 
> list at the project page.
> 
> When I started this system I was new to python (but experienced in many 
> other languages) and in the process I have done some things I am not 
> sure about. I of course test my ideas to make sure they work, but of 
> course, such tests can miss some condition in which the idiom ultimately 
> is bound to fail horribly.
> 
> So... I'm curious if something I'm doing with HDULists is ok.
> 
> Basically, we have an abstraction for our data, lets call it AstroData 
> because that's what we call it.  It contains a member which is the 
> HDUList as returned by pyfits when opening a MEF.
> 
> I also have some syntax that allows one to get an AstroData instance of 
> just sub-selections of extensions in the MEF, e.g.:
> 
> if ad = AstroData(fitsfilename) then (e.g.) ad["SCI"] returns a separate 
> AstroData instance from 'ad'. In this case I build the HDUList out of 
> the HDU entries in the original instance.  I make a python list with the 
> PHU and whatever extensions are being selected (in the sample case, 
> those where EXTNAME="SCI"), and initialize a new HDUList instance with 
> this list.  Then this unique HDUList is stored in the new AstroData 
> instance.  So each AstroData instance has it's own HDUList, but the HDU 
> entries in it are shared.
> 
> Everything works fine, to write the constructed HDUList is to save just 
> those extensions listed, as desired, and I can pass the HDUList around 
> as normal to anything that wants HDUList instances. Of course, if I 
> change any of the shared extensions, it will be seen in all HDULists 
> that refer to it, which is confusing to some but is exactly what we want 
> in this case. 
> 
> All this runs perfectly fine in practice. Does anyone know any reason 
> this might get me into trouble? It seems I have nothing to worry about, 
> but I have a nagging idea that perhaps the HDU is more tightly linked to 
> the list containing it in some way that I do not yet know.
> 
> thanks,
> craig
> 
> 
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Hi Craig,

The more appropriate list is probably astropy@scipy.org.

As to your question.  Assuming that I understand what you are 
attempting, I don't think you should run into any problems.

Chris


-- 
Christopher Hanley
Systems Software Engineer
Space Telescope Science Institute
3700 San Martin Drive
Baltimore MD, 21218
(410) 338-4338


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